1. Field of the Invention
This invention relates to digital data communications and, in particular, to methods and circuits for testing hybrid mode operation of a station in a Fiber Distributed Data Interface (FDDI-II) network.
2. Description of Related Art
FDDI (Fiber Distributed Data Interface) and FDDI-II are American National Standard (ANS) protocols for 100 Mbit/s Local Area Networks (LANs). Both protocols, FDDI and FDDI-II, are implemented on ring networks having stations which communicate over fiber optic cables.
FIG. 1 shows an example of an FDDI or FDDI-II network. The network has a trunk ring which consists of a pair of counter-rotating rings. Data travel clockwise in one ring and counterclockwise in the other. The trunk ring is formed using dual fiber optic cables 112, 123, and 113 which connect stations 101, 102 and 103.
Each station 101-106 has at least one physical layer P which comprises Physical Layer Medium Dependent (PMD) device such as a fiber optic transceiver which converts light signals to electric signals and converts electric signals back to light signals and a physical protocol device (PHY) which implements FDDI or FDDI-II protocols. Each station also has at least one Media Access Control (MAC) interface M which processes data received by a physical layer P and which provides data to be transmitted by a physical layer P. In the example shown in FIG. 1, stations 101 and 102 communicate over the clockwise rotating ring. Station 103 does not have a MAC which receives data over from the clockwise rotating ring, and therefore acts as a repeater on the clockwise rotating ring.
All stations connected directly to the trunk ring must maintain counter-rotating rings and therefore must have at least two PHY units. Single attachment stations, stations with only one PHY unit such as 104 and 105, can not be connected directly to the trunk ring, and must access the trunk ring through another station, such as station 103. A station, like station 103, which connects other stations into a trunk ring is referred to as a concentrator.
In the example ring shown in FIG. 1, stations 104, 105, and 106 are connected through concentrator 103 to the counterclockwise rotating ring. For the purposes of the rings shown, the station 106 acts as a single attachment station. However, station 106 can attach to another ring (not shown) via dual fiber optic cable 160.
FDDI provides protocols for a token ring network which transmits data packets. The data packets are made up of a variable number of 5-bit symbols. Of the 32 possible values for a 5-bit symbol, 16 values represent 4-bit data (numbers from 0 to 15), 8 values represent control signals, and 7 values represent violation symbols. Violation symbols are not used because they violate code run length and dc balance requirements.
Stations in the token ring only originate data packets after capturing a token which indicates the right to transmit. FDDI provides rules which control access to a ring by controlling when a station may capture a token. Two priorities of access are defined, "synchronous" and "asynchronous."
Every transmitted packet contains a header which indicates source and destination stations. The packets travel around the ring by being received and retransmitted by successive stations in the ring. Stations which are destination stations process the data and may alter the data packet before retransmitting.
FDDI-II enhances FDDI by allowing formation of a hybrid ring which provides both data packet service (synchronous and asynchronous service) and cycle-switched service (isochronous service). A hybrid ring is made up of two types of stations, monitor stations and non-monitor stations. Monitor stations are capable of acting as the cycle master for the hybrid ring. Non-monitor stations are only capable of being a slave station. The hybrid ring requires one monitor station to act as cycle master. All other stations in the ring, both monitor stations and non-monitor stations, are slave stations.
The cycle master produces a continuous serial bit stream of 5-bit symbols which are organized in cycles. According to the clock of the cycle master, once every 125 .mu.s, the cycle master begins a new cycle which is 125 .mu.s long. Each 125 .mu.s cycle has time slots for 1560 bytes (3120 symbols) and is preceded by a 2.5 byte preamble. Slots within a cycle are identified by timing. FIG. 2 shows the FDDI-II cycle structure. The slots are organized into 16 Wide Band Channels (WBCs) WBC 0-WBC 15 having 96 bytes size slots each, 12 byte size slots 1-12 dedicated for packet service, 2.5 bytes (5 symbols) for a preamble 201, and 12 bytes a header 202.
The cycle starts with preamble 201 and header 202 which are followed by slots for data. The header 202 is divided into fields, JK starting delimiter field 202A, cycle control field 202B, cycle sequence field 202C, programming template 202D, and isochronous maintenance channel field 202E. Wide band channels WBC 0-WBC 15 may be allocated either for packet or isochronous service. Programming template 202D indicates the allocation of each wide band channel WBC 0-WBC 15. Dedicated packet service slots 1-12 are only used for packet service. Accordingly, the hybrid ring provides a minimum of 768 kbit/sec (12 bytes/125 .mu.s) for packet service. The order of transmission in FIG. 2 is left to right starting with the top row. Thus, dedicated packet service slots 1-12 and byte sized slots of the wide band channels WBC 0 through WBC 15 are interleaved.
Isochronous service is a type of transmission that depends on identification of particular bits in a cycle. With isochronous service, N bits beginning with byte slot M in WBC number X can be allocated for isochronous transmission between particular stations on the hybrid ring. Once slots are allocated, a source station can insert information into the allocated slots and destination station can retrieve the information when the allocated slots arrive. Packet headers are not required. Isochronous service provides regular transmissions that have a well controlled rate of transfer, regardless of the work load on the ring. A 1-bit slot allocated for isochronous service provides one bit of information every 125 .mu.s or an 8 kbit/s transmission rate. Larger allocations provide faster rates.
The cycle master is responsible for generating a preamble and header at the beginning of each cycle, maintaining the timing of the cycle, and negotiating allocation of WBCs and slots of the WBCs.
FIG. 3 shows the functional units in a station which implements the FDDI-II protocol. A PMD unit 302 couples to fiber optic cables 307 and 308 and to PHY unit 303. PMD 302 converts light signals coming in on cable 307 to electrical signal which can be used by PHY 303. PMD 302 also converts electric signals from PHY 303 to light signals transmitted on cable 308. PHY 303 connects to an Hybrid Ring Control (HRC) unit 304 which routes signals to and from two types of Media Access Control (MAC). A Packet MAC (PMAC) 306 handles packet service and operates as described above. Accordingly, the PMAC handles data as transmissions on a token ring. An Isochronous MAC (IMAC) 305 handles data from the slots allocated for isochronous transmission.
Cycles travel around the ring and return to the cycle master. Along the way slave stations may insert packet and isochronous data into slots. Packet data received by the cycle master is inserted into the next available slot allocated for packet service. Isochronous data is treated differently. Typically, the time to travel around the ring will not be a multiple of 125 .mu.s, so if isochronous data were immediately repeated or transmitted by the cycle master the isochronous data would not be in the correct slot. The cycle master uses a Latency Adjustment Buffer (LAB) to delay transmission of isochronous data until the proper time slot in a cycle. To avoid loss of data, the LAB must be able to hold an entire cycle of isochronous data.
One problem with non-monitor FDDI-II stations is testing hybrid mode capabilities. Hybrid mode capabilities of monitor stations can be tested before insertion into a hybrid ring by having the monitor station operate a local ring (a small test ring). Prior art non-monitor stations can not operate a hybrid local ring and their hybrid mode capabilities cannot be tested in a local ring unless the ring includes a monitor station. After insertion into the working hybrid ring, malfunctioning stations must be identified, removed from the ring, and a new hybrid ring established. Connecting the malfunctioning station to a ring can cause data transmission problems and delays.
Testing is also a problem when different types of station are connected to a concentrator. A concentrator should not connect stations that are not capable of operating in hybrid mode into a hybrid ring. Methods of testing are needed which test the hybrid functions of a station, both internally for malfunctions and externally for FDDI-II compatibility, before inserting the station into a hybrid ring.